World-wide demand for petroleum products has increased dramatically in recent years, depleting much of the known, high value, light crude oil reservoirs. Consequently, production companies have turned their interest towards using low value, heavy crude oil in order to meet the ever increasing demands of the future. However, because current refining methods using heavy crude oil are less efficient than those using light crude oils, refineries producing petroleum products from heavier crude oils must refine larger volumes of heavier crude oil in order to get the same volume of final product. Unfortunately though, this does not account for the expected increase in future demand. Further exacerbating the problem, many countries have implemented or plan to implement more strict regulations on the specifications of the petroleum-based transportation fuel. Consequently, the petroleum industry is seeking to find new methods for treating heavy crude oil prior to refining in an effort to meet the ever-increasing demand for petroleum feedstocks and to improve the quality of available crude oils used in refinery processes.
Whole crude oil, or raw crude oil, is the general term for crude oil produced from a production well prior to any refining processes. Depending upon the geographic characteristics of the production well, whole crude oil can vary greatly in composition from well to well. Unfortunately, many newly discovered wells tend to produce whole crude oil that contains increased amounts of heavy fractions and impurities other than carbon and hydrogen. Therefore, as more of the established, more valuable oil wells become depleted, the majority of our future supply will consist of inferior crude oil.
In general, high density crude oil provides lower amounts of the more valuable light and middle distillates. Additionally, high density crude oil generally contains increased amounts of impurities, such as sulfur, nitrogen and metals, all of which require increased amounts of hydrogen and energy for hydroprocessing in order to meet strict regulations on impurity content in the final product.
Generally speaking, heavy crude oils have a low API gravity, high asphaltene content, low middle distillate yield, high sulfur content, high nitrogen content, and high metal content. These properties make it difficult to refine heavy crude oil by conventional refining processes to produce end petroleum products with specifications that meet strict government regulations.
Traditional Cracking Methods
Low value heavy crude oil can be transformed into high value light crude oil by cracking the heavy fraction using various methods known in the art. Conventionally, cracking and cleaning have been conducted using a catalyst at elevated temperatures in the presence of hydrogen. However, this type of hydroprocessing has a definite limitation in processing heavy and sour crude oil when not using of large amounts of hydrogen and/or catalysts.
Additionally, distillation and/or hydroprocessing of heavy crude feedstock produce large amounts of asphaltene and heavy hydrocarbons, which must be further cracked and hydrotreated to be utilized. Conventional hydrocracking and hydrotreating processes for asphaltenic and heavy fractions also require high capital investments and substantial processing.
Many petroleum refineries perform conventional hydroprocessing after distilling crude oil into various fractions, with each fraction being hydroprocessed separately. Therefore, refineries must utilize the complex unit operations for each fraction. Further, significant amounts of hydrogen and expensive catalysts are utilized in conventional hydrocracking and hydrotreating processes. The processes are carried out under severe reaction conditions to increase the yield from the heavy crude oil towards more valuable middle distillates and to remove impurities such as sulfur, nitrogen, and metals.
Currently, large amounts of hydrogen are used to adjust the properties of fractions produced from conventional refining processes in order to meet required low molecular weight specifications for the end products; to remove impurities such as sulfur, nitrogen, and metal; and to increase the hydrogen-to-carbon ratio of the matrix. Hydrocracking and hydrotreating of asphaltenic and heavy fractions are examples of processes requiring large amounts of hydrogen, both of which result in the catalyst having a reduced life cycle.
Consequently, it would be beneficial to crack only the heavy portion of the whole crude oil using an efficient and low cost method, so that the entire whole crude stream would consist of the more valuable light fraction, resulting in reduced downstream refining costs.
Hydrothermal Cracking—Supercritical Water
Supercritical water has been utilized as a reaction medium for cracking of hydrocarbons with the addition of an external source of hydrogen. Water has a critical point at about 705° F. (374° C.) and about 22.1 MPa. Above these conditions, the phase boundary between liquid and gas for water disappears, with the resulting supercritical water exhibiting high solubility toward organic compounds and high miscibility with gases.
However, utilizing supercritical water to upgrade whole crude oil can have serious drawbacks if the whole crude oil contains increased quantities of heavy hydrocarbon molecules. Heavy hydrocarbon molecules dissolute into supercritical water much slower than their lighter counterparts. Furthermore, asphaltenic molecules, which have tangled structures, do not untangle easily with supercritical water. Consequently, the portions of the heavy hydrocarbon molecules that do not make contact with the supercritical water thermally decompose by themselves, resulting in large amounts of coke. Therefore, if the whole crude oil contains increased quantities of heavy hydrocarbons, reacting the whole crude oil with supercritical water using current methods leads to accumulation of coke inside the reactor.
When coke accumulates inside a reactor, the coke acts as an insulator and effectively blocks the heat from radiating throughout the reactor, leading to increased energy costs, since the operator must increase the operating temperature to offset for the build-up. Furthermore, accumulated coke can also increase the pressure drop throughout the process line, causing additional increases in energy costs.
One possible solution to prevent coke build-up is to increase the residence time of the whole crude oil within the reactor for dissolving whole parts of crude oil and decrease the temperature of the reactor; however, the overall economy and upgrading performance of the process would be reduced. Additionally, improvements in reactor design could be helpful; however, this would require large expenditures in design costs and might ultimately not prove beneficial. Therefore, there is a need for a process to facilitate the efficient contacting of heavy oil with supercritical water, which does not result in large amounts of coke or substantial increases in operating costs.
Enhanced Oil Recovery
Enhanced Oil Recovery (EOR) is a generic term for techniques for increasing the amount of oil that can be extracted from an oil field. Using EOR, approximately 30-60%, or more, of the reservoir's original oil can be extracted compared with 20-40% using primary and secondary recovery. Typical fluids used for EOR include gases, liquids, steam or other chemicals, with gas injection being the most commonly used EOR technique.
In a gas type EOR, gas such as carbon dioxide (CO2), natural gas, or nitrogen is injected into the reservoir, whereupon it expands and thereby pushes additional crude oil to a production wellbore. Moreover, the gas dissolves in the crude oil, which lowers the crude oil's viscosity and improves the flow rate of the crude oil through a transferring line.
When CO2 is pumped into an oil reservoir at a pressure sufficient to make it as dense as the oil in the reservoir, the CO2 may become miscible with the oil. The pressure at which miscibility is first achieved is called the minimum miscibility pressure (MMP). At or above its MMP, CO2 becomes an ideal solvent for oil, and because of this, it displaces oil from the reservoir much more efficiently than water. It picks up lighter hydrocarbon components, swells the total volume of oil, and reduces its viscosity so that it flows more easily.
CO2 is currently one of the most promising crude oil recovery fluids because the dissolved CO2 can be easily separated from the recovered crude oil after production by depressurization. Of course, the solubility of CO2 in crude oil depends heavily on pressure, temperature, gas to oil ratio and composition of the crude oil. However, the simplest way to control the phase behavior of CO2 and crude oil is to vary the pressure. At low pressures, CO2 shows very low solubility toward crude oil, in particular, the heavy fraction. Additionally, the dissolving of CO2 in the crude oil causes the crude oil to swell, resulting in increased solubility of asphaltenic species that may be in the crude oil.
As stated earlier, one of the shortcomings of contacting a high density whole crude oil with supercritical water was the production of large quantities of low value coke. This coke production was caused by the inability of the supercritical water to effectively penetrate throughout the high density whole crude oil, particularly the heavy fraction of the whole crude oil. However, since CO2 dissolved in crude oil causes the crude oil to swell, and thus become less dense, combining a CO2 EOR method with supercritical water allows for upgrading the whole crude oil without the production of considerable amounts of coke by facilitating dissolution of the heavy fraction into the supercritical water.
Processing an entire stream of whole crude oil is economically unfeasible as the throughputs are too high. Therefore, it would be desirable to have a simple and economical process to combine a CO2 EOR method of recovery with a supercritical water cracking method, while contacting only the heavy portion of the whole crude oil in order to limit coke conversion, increase overall well production, and produce a final crude oil that is mostly higher value, light fraction.
Furthermore, it would be desirable to have an improved process for upgrading whole crude oils with supercritical water fluid that requires neither an external supply of hydrogen nor the presence of an externally supplied catalyst. It would be advantageous to create a process and apparatus that allows for the upgrade of the whole crude oil, rather than the individual fractions, to reach the desired qualities such that the refining process and various supporting facilities can be simplified.
Additionally, it would be beneficial to have an improved process that did not require complex equipment or facilities associated with other processes that require hydrogen supply or coke removal systems so that they may be implemented at the production site.